Each year millions of dollars are spent on lithium batteries for use in portable electronics equipment. Because of their superior rate capability and service life over a wide variety of conditions, lithium batteries are the power source of choice for many equipment applications. There is no convenient method of determining the available capacity remaining in partially used lithium batteries. Hence, users do not take full advantage of all the available battery energy. In order to maintain readiness, users currently replace batteries on a conservative schedule. This practice results in the waste of millions of dollars in battery life or energy every year.
It is well documented and accepted that the available energy in a lithium battery is a function of the conditions to which the battery has been subjected. Capacity remaining is a complex function of current drain, temperature and time. Therefore, a reliable method of predicting remaining capacity has been actively sought. External monitoring/testing devices are available for most battery systems. However these devices are, in many cases, not portable and imprecise. Therefore a continuous internal means of determining capacity is desirable. Lithium/sulphur dioxide cell behavior at different temperatures and current drains have been examined. This examination has resulted in the establishment of discharge efficiency formulas. Utilization of these formulas has given rise to a capacity prediction algorithm.
The ability to monitor a battery's state of charge and display this to the user in real time allows for efficient utilization of battery energy. A simple technique for determining the state of charge of a battery is performed by measuring the voltage across the battery and relating this to the battery's state of charge. This method of determining the battery state of charge does not apply for many battery systems, particularly high power/high energy systems where knowledge of the state of charge is most important.
Another technique for determining the state of charge is performed by monitoring the energy removed, then by subtracting this value from the total capacity, the state of charge can be determined. This method is valid for all battery systems. However, large errors can occur whenever the battery is discharged at conditions different from the calibration standard.
Inexpensive accurate state of charge indicators for high power high energy batteries are not available to date. Accurate state of charge indication can only be achieved by taking the discharge rate and temperature into account.